Reverting, then, to the " idea" of a species, as involving descent from a common, or at any rate from a similar parentage, in all the in dividuals composing it, we have to aim at ascertaining, in the case of two or more beings whose specific identity or distinctness is a question for our determination, whether their characters are presented so fixedly and deter minately in all the individuals of the same and of successive generations, as to justify us in believing that they have been thus preserved through all time, and under all changes of external conditions. According to the amount and correctness of our information upon this question, will be the validity of our specific distinctions ; on the other hand, according to the hastiness and crudity of our decision, will be its liability to be overthrown by subsequent researches. Of course, where the progeny of any known stock can be traced through a long period of time, and under great varieties of external conditions, and their successive variations have been noted, this evidence must outweigh every argument founded upon the supposed importance of the characters which are found to undergo modification. But such opportunities are too frequently wanting ; and the naturalist is obliged to have recourse to means of discrimination which are less certain, but which will frequently conduct him, pro vided that his researches have been sufficiently extensive, to a satisfactory conclusion. The great point at which he should aim, is the assemblage of as many forms as possible of each type ; and having done so, he will care fully compare them with each other, for the sake of determining whether the supposed specific characters are constant and well marked throughout, or whether they tend to run together by intermediate gradations. If the first of these should prove to be the case, great confidence may be entertained of their genuineness; but if the second, we may feel an almost certain assurance of their invalidity. Thus, to revert to the case of the apple and the pear, the persistence of their distinctive characters through all the numerous varieties of each, renders it almost certain, that in all other varieties which may hereafter present themselves, the same constancy will obtain; and that it has obtained during the entire suc cession of generations of pears and apples, from the time of their first propagation. But let us take an opposite case. Two are brought together from different parts of the great Southern Ocean, the one of which has the edges of the valves of the shell thrown into deep plications, whilst in the other they are quite smooth. Now in most other Bivalve Mollusca, such a difference would be justly admitted to afford a valid specific character and the conchologist who had only these two shells before him, would be justified, by the usual rules of the science, in ranking each as a distinct specific type. But as his col lection extends, intermediate forms come into his possession ; and at last he finds that he can make a continuous series, passing, by the most gradual transition, from the smoothest to the most deeply plicated form. Thus, then, the supposed validity of this distinction is altogether destroyed ; and it becomes evident that the most plicated and the smoothest of these Terebratulce must be regarded as be longing to one and the same species, notwith standing the marked diversity of their extreme forms.
Hence, whilst new types are continually being discovered, the progress of research is tending to diminish the number of species previously enumerated ; for there are many groups in which an immense reduction has been effected, by bringing together all those which are found to be nothing else than suc cessive stages of the same individual, and by ranking under one designation all those which are either known or strongly suspected to he mere varieties, resulting from the direct influ ence of external conditions upon themselves or upon their ancestors, or produced through the obscurer operation of these influences on the act of generation. Frequently it is found that forms which have even been accounted generically distinct, are in reality specifically identical. Thus it has been shown by Pro fessor Henslow, that the " rust of corn" (Uredo rztbigo) is but an earlier form of the " mildew " (Puccinia graminis); the one form being capable of development into the other; and the fructification characteristic of the two supposed genera having been produced from the same individual. And it is asserted by
Fries, that out of a single species of Thelephora, more than eight genera of Fungi have been con structed by various authors. So among higher plants, the invalidity of the generic distinctions on which reliance is usually placed, has been shown, so far as the Orchideous tribe is con cerned, by the fact that the same individual has borne the flowers and pseudo-bulbs usually accounted characteristic of three distinct genera, and that another individual has pre sented the character of a fourth.* So in the animal kingdom, it has been shown by Pro fessor Milne Edwards, that the polypidom of Tubulipora verrucosa, according to the circum stances under which it grows, may present the characters of three other reputed genera. If it be attached to a plane surface, as the expanded lamina of a sea-weed, it remains circular, and increases with great regularity, constituting the Madrepora verrucosa of Fabricius. If it cluster round the cylindrical and branching stem of a Fucus, it increases irregularly, and assumes the form of the Millepora tubulosa of Ellis. If its development in any direction be checked by a mechanical obstacle, the form of the mass will again be changed, and its tubes will be recurved backwards; a character on which Lamouroux founded his genus Obelia. Sometimes on the very same polypidom, we find one portion whose disposition corre sponds with that of Millepora tubulosa, and another which, if detached, would be con sidered a specimen of Obelia tubulifera.% Many similar cases might be quoted ; all of them showing, not that there is any real con fusion amongst species and genera, but that naturalists have too often assumed variable and non-essential characters as the basis of their systematic distinctions, in ignorance of those which are fixed and determinate. Thus, in the case in question, it is on the structure of the animal, not on the form of the poly pidom, that the modern Zoophytologist places his chief reliance ; and a knowledge of this would have prevented the assignment of the varieties of coral, formed by one and the same kind of animal, to three different genera. So among Mollusca, it has been shown by Mr. Gray t that a large number of species have been formed, in consequence of the variations pre sented by the shells of the same species at different periods of life, or developed under different circumstances. The change from salt or fresh water to brackish, or from brackish to salt or fresh, which many species are able to sustain, appears to have a con siderable influence on the form of their shells ; thus Professor E. Forbes has shown that certain Pah:dinar and Naticce, which are found in successive tertiary strata in the isladd of Cos, associated in some cases with decidedly fresh-water, and in others with decidedly marine, testacea, are probably to be regarded as varieties of the same species, notwith standing that they would be regarded by conchologists as distinct ; gradations being traceable between one form and another, and the changes being of a kind which are known to take place among fresh-water mol lusca.$ As an example of the mode in which the philosophic zoologist proceeds in his exami nation of a doubtful case of unity or diversity of species, in a group more closely related to man, we shall consider the question of the relations of the several races of Dogs to each other. Every one is familiar with the fact that numerous breeds of dogs exist in almost every part of the world inhabited by civilised man, distinguished from each other by well-marked peculiarities, which appear to be transmitted continuously from parent to offspring, and thus to possess a claim to rank as specific distinctions. These dif ferences extend to stature, form, proportions, swiftness of foot, colour and texture of hair, acuteness of sensations, intelligence, and at tachment to man ; and they are particularly well marked in the conformation of the cra nium, the part to which the anatomist first looks for his distinctive characters.